1. Field of the Invention
The present invention relates to the production of lower olefins such as ethylene, propylene, butenes and pentenes from higher hydrocarbons by contacting the higher hydrocarbon feed with a particular ZSM-5 zeolite catalyst which has a surface Si/Al ratio of 20 to 60, which contains phosphorus and which has been steam treated after phosphorus addition The contact is carried out at conditions which favor lower olefin formation including high space velocity of at least 50 hr.sup.-1 WHSV, high temperature, low conversion per pass and low hydrocarbon partial pressure.
2. Description of the Prior Art
Methods are currently known for the production of commercially important olefins such as ethylene, propylene, butenes and pentenes from paraffinic feed materials. Such methods include steam cracking, propane dehydrogenation and various refinery catalytic cracking operations.
Each of these procedures has certain disadvantages. For example, propylene yields from steam cracking are not very high and are not substantially improved by recycling. Purification of non-propylene products is required which is costly or such products have only fuel value.
Propane dehydrogenation processes are characterized by rapid catalyst coking requiring frequent, costly regenerations. Also, reasonable conversions require sub-atmospheric pressures, and propane is difficult to separate from propylene.
Propylene supplies from catalytic conversions are uncertain. Transportation and purification are significant problems.
Methods are known for the production of lower olefins from higher hydrocarbon feedstocks. Copending application Ser. No. 07/500,172 filed Mar. 27, 1990 describes an improved process for the formation of lower olefins from mixtures of paraffins and olefins using zeolite catalysts including phosphorus containing zeolites.
European 0 109059 shows the production of lower olefins from higher hydrocarbons at high temperature and high space velocity using zeolite catalysts.
U.S. Pat. Nos. 3,972,832 and 4,044,065 show hydrocarbon conversions using a phosphorus-containing zeolite such as ZSM-5.
U.S. Pat. Nos. 4,356,338 and 4,423,266 show that zeolite catalysts can be advantageously treated with phosphorus and/or steam.
U.S. Pat. Nos. 4,559,314 and 4,784,747 show that zeolite activity can be improved by forming the catalyst into a composite with a binder such as alumina and steaming the composite.